The use of gas turbine engine powered aircraft has greatly increased over the course of the past few decades, especially over urban areas. As a result, the United States government has imposed increasingly more stringent gas turbine engine noise standards meant to reduce noise pollution from airports and overhead passing jet aircraft. The latest standards are the federal "stage three" requirements.
Most older aircraft do not meet the stage three noise requirements. Some, like the common, business-class Gulfstream.RTM. GII, GIIB, and GIII jets, have low bypass ratio turbofan engines (with extremely high jet velocities), and miss the mark by a wide margin. Other, somewhat larger jets, such as the MD-80.RTM. manufactured by the Boeing Company, have quieter, high bypass ratio engines. However, even with these quieter engines, many such aircraft are still too noisy.
For example, the MD-80.RTM. jet has two fuselage mounted Pratt & Whitney.RTM. JT8D-219 gas turbine engines. Tests have shown that while some of the stage three requirements are met for these engines, some are not. More specifically, an MD-80.RTM. jet misses the stage three requirements for sideline and flyover noise by as much as about 0.3 dB. Although this is a fortuitously low amount, it still means that these engines need to be provided with some sort of noise reduction device for the airplane to meet federal requirements.
Unfortunately, many gas turbine engine noise suppressors are designed for noisy, low bypass ratio engines, wherein the gas turbine engine noise has to be reduced by as much as ten to twelve decibels. Although such a level of noise reduction would be advantageous for any type of airplane, it typically comes at the expense of a loss in cruise performance.
For example, to alleviate its noise problems, a Gulfstream.RTM. jet may be advantageously equipped with aft mounted mixer/ejector-type noise suppressors. These types of noise suppressors typically comprise an ejector shroud coupled to a lobed mixer, which replaces a gas turbine engine's existing round exhaust nozzle. Engine exhaust passes out the engine proper, through the lobed mixer, and into the ejector shroud. At the same time, cooler, lower velocity, ambient air outside the engine passes over the lobed mixer to enter the ejector shroud via spaces between the shroud and mixer. The lobed mixer causes the ambient air to quickly mix with the engine exhaust, creating a uniform flow by the time the combined gasses exit the ejector shroud, and cooling and slowing the engine exhaust. This lowers the engine's noise output, and actually increases available thrust. However, at the same time, because the lobed mixer extends into the exhaust stream, engine drag is slightly increased, resulting in a cruise performance loss of up to about seven percent.
Although such a loss is perfectly acceptable for smaller, private aircraft like the Gulfstream.RTM., it would be very disadvantageous for larger, commercial airplanes like the MD-80.RTM., where a seven percent loss in cruise performance would cost an airline company a lot of money in extra fuel and shortened airplane range. Moreover, because the MD-80.RTM. has relatively high bypass ratio engines (with subsequently lower jet velocities), noise suppressors designed for low bypass ratio engines (with supersonic jet velocities), such as those described above, would likely not work very well there with. For example, they might not absorb the necessary sound frequencies, and would likely not produce a proper exhaust/ambient air mixing effect.
Additionally, because MD-80.RTM. airplanes and similar aircraft are already so close to meeting the stage three requirements, and are otherwise perfectly functional, there is little need to outfit them with expensive noise reducers that would require replacing additional components of the airplane's engines. For example, aft mounted mixer-ejector noise suppressors are not compatible with "post-exit-" or "bucket-" type thrust reversers, such as those found on the Pratt & Whitney.RTM. JT8D-219 gas turbine engines.
By way of explanation, a thrust reverser is a mechanical device that is deployed to redirect exhaust flow from an aircraft's gas turbine engines, typically just subsequent to the aircraft landing. All thrust reversers are normally deployed during the landing sequence, after the nose wheel has touched down (usually called the "rollout"). Thrust reversers can greatly reduce the length of runway necessary to bring the aircraft to taxi speed, and they are also used when adverse weather conditions, such as ice on the runway, may cause the aircraft's brakes to be ineffective. As shown in FIGS. 1 and 2, conventional post-exit thrust reversers 20 redirect the engine exhaust after it leaves a round exhaust nozzle 22 via a set of "bucket" doors 24, 26 that open up behind the engine.
As should be appreciated, mixer/ejector noise suppressors need to be attached to the exit end of a gas turbine engine to function. In the case of the Pratt & Whitney.RTM. JT8D-219 gas turbine engines, such noise suppressors would have to be affixed to the engines in roughly the same spaces occupied by the engines' post-exit thrust reversers. Of course, this is impossible, and either the post-exit thrust reversers have to be replaced (at additional, unnecessary expense), or a different type of noise suppressor has to be used.
Accordingly, it is a primary object of the present invention to provide a unique contoured thrust reverser and lobed nozzle noise suppressor for gas turbine engines that lowers engine noise to a moderate extent while minimizing airplane cruise performance loss.
Another primary object is to provide a unique thrust reverser and noise suppressor mechanism that is compatible with an engine's existing, post-exit type thrust reverser via a minimal replacement of existing thrust reverser parts.
It is a more specific object to provide a post-exit thrust reverser with blocker doors having aft lobed sections that nestle into the complementary shaped lobes of an adjacent lobed nozzle, while stowed prior to deployment.
Yet another object of the present invention is to provide a unique pair of thrust reverser blocker doors that are aerodynamically compatible with a lobed nozzle noise suppressor.